Greywacke is the dominant rock type in the Cook Strait, as well as the rest of the country; it forms the backbone of the country. The material Greywacke is comprised of a sedimentary rock formed 200 million years ago. Mud and sand settled deep in oceanic trenches; a process of compression, tectonic uplift, folding, layering and further compression finally realised the eroded Greywacke forms we see today throughout the North Island.

The Cook Strait appears to have been formed by eroding then flooding of a land bridge in the late Tertiary period (66 million to 2.58 million years ago). The sea withdrew several times during the Pleistocene (2,588,000 to 11,700 years ago) Glaciations. This is the period of the world wide lowering of the sea due to huge amounts of ice on the land. But the sea always returned, and it is believed that during the last glaciations the sea dropped about 120 metres joining the North and South Islands leaving a large sound in the place of the Strait.

Wind in central New Zealand comes from atmospheric systems from the Tasman Sea flowing from west to east. These systems occur around Antarctica at about the 45°S-the mid latitudes. These systems occur because of the succession of high pressure regions (anticyclones) separated by regions of low pressure (depressions or troughs).

Winds will often flow towards New Zealand from a southeast and northeast direction. New Zealand lying perpendicular to them becomes a large obstacle. The air that flows around NZ is directed by the land, and some is channelled into the Cook Strait. It seems that much of the air is guided by the high ground into the Narrows of the Cook Strait resulting in strengthened air flow in and around Wellington. The flow in Tasman Bay and Golden Bay is different because of the high ground of D’Urville Island and the west coasts of Malborough Sound. Northerlies and northeasters are winds that dominate in Nelson because of this.

The other dominant winds in Cook Strait are Southeasters. Air that rushes up from the south to the south end of the Tasman Sea maintains its cohesion. The mountains funnel the air into the Strait and it rushes up and out to the Tasman sea. Air flow is changed from the southeaster flow to southerly as it goes through the Narrows and then changes back to southeaster as it flows in the Tasman Sea.